The present invention relates to high frequency loudspeaker horns. More specifically, it relates to midrange and high frequency horn termini, horn enclosure and baffle acoustic interactions, and reducing subsequent horn mouth and cabinet diffraction effects.
The current availability of coaxial or extended frequency range compression drivers promote the use of single horns for loudspeakers whereas in the past two or more separate horns were typically required. A horn using the tractrix expansion formula has proven to be particularly adept at propagating an extended frequency response and presenting a wider coverage pattern in a relatively compact size, as compared to an exponential horn of the same overall low frequency cutoff (Fc). The use of a single horn for upper frequency reproduction is preferable to using multiple frequency-divided horns as it presents a single point acoustic source to the audience.
An important consideration in selecting an appropriate horn mouth size and shape in such an application as described above is that horns of this type are typically mounted or placed on top of (or in close proximity) a bass cabinet, which is usually wider than the high frequency horn mouth. Being close to a sharp-edged planar element such as embodied by a bass cabinet and/or any other edged boundary will propagate aberrant diffraction events. This presents the problems associated with baffle diffraction (the baffle being the front-facing mounting panel to which the high frequency horn is typically mounted) and any resultant cabinet diffraction around the mounting baffle and associated enclosure, if any. Since the high frequency horn or cabinet housing associated with the high frequency horn is usually on top of the bass enclosure, the diffraction experienced is usually more acute to each side of the mounting baffle or horn mouth.
It is well known in the art that transitions in the shape of a loudspeaker enclosure such as edges or slots act as additional acoustic sources. Sound wave arrivals from these acoustic sources are typically delayed behind the primary wave and are usually reversed in polarity. Additionally, it is also well known in the art that directivity is not governed exclusively by mouth or horn shape, but that diffraction from the mouth and from intermediate transitions can also influence the qualities of both response and directivity as much as the interior horn shape. Naturally, these aberrant elements should be avoided whenever possible.
A loudspeaker enclosure shape which specifically reduces horizontal cabinet diffraction was disclosed in the text book “Acoustical Engineering”, by Harry F. Olson (D. Van Nostrand, Princeton, N.J., 1957) page 169 figure 6.45, which employs receding vertical baffle angles on each front corner of a planar front baffle in which the sound producing source was centrally located. The similarities between the cited prior art and a high frequency horn placed on top of a large bass enclosure can be seen to form a typical loudspeaker shape, that is, a generally rectangular parallelepiped form which is subject to horizontal cabinet diffraction effects in combination with the possible addition of horn mouth diffraction.
The properly designed tractrix horn mouth with expanded horizontal dispersion characteristics tends to terminate with its horn walls substantially perpendicular to the horn pathway axis. Theoretically, the waveform propagation of such a device is hemispherical rather than planar. The perpendicular horn terminus side walls tend to reduce diffraction in that there is no abrupt corner for the waveform to act as an additional sound source. When such a horn is mounted to a flat baffle, a typical application, as when mounted inside an enclosure to be placed on top of a low frequency cabinet, the top enclosure sides will typically have sharp corner edges and will tend to introduce aberrant waveform propagation behavior. The production of a high frequency horn enclosure without acoustically sharp edges may also introduce economic concerns and most manufacturers accept the deleterious effects of diffraction associated with the more economically constructed forms.
Middle range frequency horns are typically produced with an integral flat flange-type mounting frame adjacent to the horn mouth for attaching the horn to a flat baffle. In addition to adding surface area requirements when attaching such a horn to a cabinet or baffle as is common to the art, horn mouth terminus-based horn mounting flanges tend to act as baffle surface interruptions, which may not be considered critical for midrange frequency reproduction, but may adversely effect high frequencies due to the very short wavelengths involved.
The formulas for calculating the values of tractrix horns are well known in the art. Such examples can be found in the magazine article “The Tractrix Horn Contour”, by Bruce C. Edgar, Speaker Builder magazine, February 1981, and a practical how-to tractrix horn design is presented in another article by the same author titled “The Edgar Midrange Horn”, Speaker Builder magazine, January 1986. The tractrix expansion rate is preferred in the current invention due to its substantially 90 degree side wall terminus plane compared to the pathway axis, however, the current invention is not limited to its exclusive use, and other expansion rates may be used as desired. In addition to the previously mentioned attributes of the tractrix horn capable of the of the frequency range presented above having a relatively compact size and wide bandwidth capability, the propagation characteristics of the tractrix flare are conducive to being readily enhanced by the current invention.
Essentially the same diffraction-producing conditions exist with the application of a high frequency horn being centrally mounted on a front baffle (as is typical of most applications which employ a midrange horn), and by logical extension, the benefits of reduced horizontal cabinet diffraction can be achieved by applying the same solution as shown in the Olson prior art device mentioned previously. However, in the case of a free-standing upper frequency horn, that is, an application where no top enclosure is desired or present for reasons of cost or aesthetic considerations, a cabinet-based diffraction reducing solution such as in the cited prior art is not possible.
It is therefore desirable to produce a wide bandwidth (midrange and high frequency) horn which does not require a traditional baffle mount and which preferably comprises its own baffle while reducing the deleterious effects of horizontal horn mouth and/or cabinet diffraction to practical or negligible limits, while providing a variety of mounting options and enclosure methodologies to be easily and economically realized.